[3.x] Bullet support improvements

[m4nu3lf]

Describe the project you are working on

Physics-based sandbox with vehicle construction and destruction.

Describe the problem or limitation you are having in your project

• Cannot have the physics engine run at a higher frequency than scripts which is helpful to get a more accurate simulation without the weight of running all scripts at that frequency.
• Adding and removing collision shapes to rigid bodies is very slow.
• Cannot activate joint force feedback (useful for destruction).

Describe the feature / enhancement and how it helps to overcome the problem or limitation

• Expose a substep parameter to be used by Bullet.
• Optimize how collision shapes are generated in Bullet.
• Expose the force feedback functionality in Godot Joints.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

I have locally patched Godot with the following features:

• Added a "substeps" project parameter and implemented support for it in the bullet wrapper. Not that _integrate_forces() will still be called at every substep, and contacts will be reported for every substep.
• Changed the way collision shapes are re-generated in Bullet. Instead of doing that every time one collision shape is changed (which is O(N^2) if you change all collision shapes), I mark the body as "dirty" and then reload all the collision shapes when required. Now changing all collision shapes in a single frame is O(N) in the number of collision shapes. I also first remove all bodies that require reloading in the dynamics world, change the shapes and only then add them back. This should lower the number of pairs being updated by Bullet in some cases.
• Allowed enabling force feedback in joint nodes.

Additionally:

• I fixed the one-frame lag in the Bullet.
• Some other minor optimizations.
• I fixed area space override for bodies with two or more collision shapes.

Given I already have most of the changes it would be a matter of polishing the code and creating a PR. Then I would need some help with testing (just to make sure I'm considering everything).

If this enhancement will not be used often, can it be worked around with a few lines of script?

No, this requires to be implemented at the engine level.

Is there a reason why this should be core and not an add-on in the asset library?

This can only be implemented at the engine level.

[Calinou]

Could substeps also be implemented in GodotPhysics? They sound useful for racing games where simulation often runs at 200 Hz or more.

If this is possible, I assume substeps would also have to be implemented in 2D. It can surely help against tunneling issues while not increasing CPU usage as much as it would otherwise do when increasing Physics FPS.

[m4nu3lf]

Could substeps also be implemented in GodotPhysics? They sound useful for racing games where simulation often runs at 200 Hz or more.

If this is possible, I assume substeps would also have to be implemented in 2D. It can surely help against tunneling issues while not increasing CPU usage as much as it would otherwise do when increasing Physics FPS.

It should be possible to implement it for the other physics engines, both 2D and GodotPhysics. I haven't looked into it. But the basic idea is you just call "step()" multiple times internally. Bullet has some optimizations for substeps and it handles them internally. I could look into it, but given I have the code for bullet almost ready would probably be better to add support for Bullet first?

[Calinou]

I could look into it, but given I have the code for bullet almost ready would probably be better to add support for Bullet first?

Sounds good to me, but this feature will have to be ported to GodotPhysics so it can be added in master and preserved in 4.0.

[m4nu3lf]

I will have a look and see if the implementation is trivial, in which case I will just make the change. Question about the impulse feedback for joints (Which could also be implemented for GodotPhysics). How would the PhysicsServer API look like? I was writing something like this:

    virtual void joint_enable_feedback(RID p_joint, bool p_enabled) = 0;
    virtual Vector3 joint_get_applied_linear_impulse_body_a(RID p_joint) const = 0;
    virtual Vector3 joint_get_applied_torque_impulse_body_a(RID p_joint) const = 0;
    virtual Vector3 joint_get_applied_linear_impulse_body_b(RID p_joint) const = 0;
    virtual Vector3 joint_get_applied_torque_impulse_body_b(RID p_joint) const = 0;

[Calinou]

Question about the impulse feedback for joints (Which could also be implemented for GodotPhysics). How would the PhysicsServer API look like? I was writing something like this:*

The API looks reasonable to me, but I'm not well-versed in that part of the physics engine. I also wonder if the joint positions should be something like "start" and "end" instead of "a" and "b" – I don't know how they're called elsewhere in the engine.